1. Field of the Invention
The present invention relates to an electromagnetic actuator adapted to utilize as actuating force magnetic attractive force produced by energizing a coil; and to a fluid-filled active vibration damping device employing the same.
2. Description of the Related Art
In the field of actuators utilized for example as actuating force generation sources in one class of vibration damping devices, namely fluid-filled active vibration damping devices, the use of electromagnetic actuators is known. An electromagnetic actuator includes a stator provided with a coil, and a movable member capable of displacement in the axial direction of the coil with respect to the stator; through the action of a magnetic field generated by energizing the coil, attractive force arises between the movable member and the stator, causing the movable member to experience relative displacement in the axial direction with respect to the stator. Additionally, an oscillator member attached to the movable member is linked by a supporting rubber elastic body to an annular support member attached to the stator, and when energization of the coil ceases and the attractive force disappears, the movable member is restored to its original position with respect to the stator by the recovery force of the supporting rubber elastic body. It is accordingly possible to bring about oscillatory displacement of the oscillator member through ON/OFF control of current through the coil. Such an arrangement is disclosed in Japanese Patent No. 4016343.
By providing a missing section in the magnetic path formed around the coil in order to define a magnetic pole defining portion, attractive force arising from the magnetic force acting between the movable member and the stator is exerted between this magnetic pole and the ferromagnetic movable member. Because the magnitude of the attractive force arising from magnetism in this way is inversely proportional to the square of the distance separating the magnetic pole defining portion and the movable member in the axial direction, the smaller the distance separating them, the more efficiently the actuating force will act on the movable member.
However, as noted earlier, because attractive force is inversely proportional to the square of the separation distance, if the separation distance is small, there is a risk that the movable member will become difficult to control owing to the sharply increased attractive force, causing problems such as the movable member striking against other components as a result of excessive displacement, and associated noise. It is therefore necessary to ensure a sufficiently large distance separating the movable member and the magnetic pole defining portion, and this made it difficult for actuating energy to be realized efficiently.